The plasma technologies are widely used in metal surface engineering processes. Basically, these treatments improve the mechanical, tribological, and chemical properties of the material such as wear resistance, hardness, fatigue resistance, friction, and corrosion resistance. In this work, a comprehensive study of the influence of the microstructure on hardness of AISI P20 steel treated at different temperatures and times by pulsed plasma nitriding is reported. The processes were done by using a pulsed plasma industrial system. The samples were characterized by nano-indentation (hardness), x-ray diffraction (XRD), scanning electron microscopy (SEM), and x-ray dispersion spectroscopy (EDS). At lower treatment temperatures (360 degrees C), a high density of small lamellar precipitates, constituted by more epsilon-Fe2-3N phase than gamma(')-Fe4N phase, is formed. At intermediate treatment temperatures (480 degrees C), big lamellar precipitates, constituted by more gamma(')-Fe4N phase than epsilon-Fe2-3N phase, are formed at grain boundary. At higher treatment temperatures (520 degrees C), the nitrided layer does not contain lamellar precipitates and it is only constituted by alpha-Fe phase saturated in nitrogen. Hardness depends on size, shape, and distribution of precipitates and crystalline phases (microstructure). The higher hardness values are obtained when more and smaller lamellar precipitates are presented and constituted by more epsilon-Fe2-3N phase.